Gas discharge lamps, in particular Ultra High Pressure (UHP) lamps, are nowadays used in beamers and projection television systems. Such applications require good lighting control in order to provide a suitable lighting condition. The quality of the output light depends inter alia on a current profile of a current supplied to the lamp. A lamp characteristic of the UHP lamp, in particular a dynamic characteristic, changes over its lifetime. Therefore, accurate control of the current through the UHP lamp over its entire lifetime may be difficult.
It has been proposed to use a microprocessor-based system to implement lighting control. For example, a strategy using iterative learning may be employed in a control method. In such a method, a control signal for a repeating task may be updated iteratively such that a difference between a desired characteristic of the lamp and a corresponding actual characteristic diminishes.
In the above-mentioned iterative learning control method a lamp current may be employed as the controlled lamp characteristic. In order to determine the actual lamp current it is known to incorporate a shunt, in particular a resistive shunt, in the lamp driver circuit. The lamp driver circuit, however, may be a switched mode power supply (SMPS) such as a buck converter or a boost converter. For controlling a SMPS an output current is to be determined. Thereto it is known to incorporate a further resistive shunt in the lamp driver circuit. As two resistive shunts are present, e.g. placed in a ground line, it is known to perform a differential voltage measurement on one of the resistive shunts in order to determine a voltage drop across the resistive shunt. Such a differential voltage measurement requires signal processing circuitry, e.g. comprising a differential amplifier. Thus, the straightforward current measurement using two resistive shunts results in a complicated and expensive lamp driver circuit.